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Function Parameters and Return Values

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In this section, we will delve into the details of defining and using function parameters and return values in ALGOL. Understanding these concepts is crucial for writing modular and reusable code.

Key Concepts

  1. Function Parameters: Variables that are passed to a function when it is called.
  2. Return Values: The value that a function returns after execution.

Function Parameters

Types of Parameters

  1. Value Parameters: These are the most common type of parameters. The function receives a copy of the argument's value.
  2. Reference Parameters: These allow the function to modify the argument's value directly.

Syntax

The general syntax for defining a function with parameters in ALGOL is as follows:

procedure FunctionName(parameter1: type; parameter2: type);
begin
    // Function body
end;

Example

Let's look at a simple example where we define a function to add two numbers:

procedure AddNumbers(a: integer; b: integer) -> integer;
begin
    AddNumbers := a + b;
end;

In this example:

  • a and b are value parameters of type integer.
  • The function returns an integer value which is the sum of a and b.

Return Values

Syntax

To specify a return value, you use the -> symbol followed by the return type in the function definition.

Example

Continuing from the previous example, the AddNumbers function returns an integer value:

procedure AddNumbers(a: integer; b: integer) -> integer;
begin
    AddNumbers := a + b;
end;

Using the Function

To use the AddNumbers function, you would call it and store the result in a variable:

integer result;
result := AddNumbers(5, 10);
print(result);  // Output: 15

Practical Exercises

Exercise 1: Subtract Numbers

Task: Write a function SubtractNumbers that takes two integers and returns their difference.

Solution:

procedure SubtractNumbers(a: integer; b: integer) -> integer;
begin
    SubtractNumbers := a - b;
end;

Usage:

integer result;
result := SubtractNumbers(10, 5);
print(result);  // Output: 5

Exercise 2: Swap Values

Task: Write a function SwapValues that takes two reference parameters and swaps their values.

Solution:

procedure SwapValues(ref a: integer; ref b: integer);
integer temp;
begin
    temp := a;
    a := b;
    b := temp;
end;

Usage:

integer x, y;
x := 5;
y := 10;
SwapValues(x, y);
print(x);  // Output: 10
print(y);  // Output: 5

Common Mistakes and Tips

  1. Forgetting to Specify Return Type: Always specify the return type using the -> symbol.
  2. Incorrect Parameter Types: Ensure that the types of the parameters match the types of the arguments passed.
  3. Modifying Value Parameters: Remember that changes to value parameters do not affect the original arguments.

Summary

In this section, we covered:

  • The difference between value and reference parameters.
  • How to define and use function parameters in ALGOL.
  • How to specify and use return values.
  • Practical exercises to reinforce the concepts.

Understanding function parameters and return values is essential for writing effective and modular code in ALGOL. In the next section, we will explore recursive functions, which build upon these concepts.

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